1. Field of the Invention
The present invention relates to an image sensor, and more particularly, to an image sensor which includes multiple imaging planes.
2. Description of the Prior Art
Typical charge-coupled device (CCD) image sensors are formed on flat, circular semiconductor substrates known as wafers. Image sensor elements are fabricated on either a top or bottom surface of the wafer by means of various doping layers of several microns in depth. Upon completion of the image sensor elements, the die is cut from the wafer and mounted in a package where either the top or bottom surface is exposed for illumination.
In U.S. Pat. No. 4,031,315, there is shown a CCD image sensor which comprises image sensor elements arranged in matrix form on one surface of the image sensor. In one embodiment, the image sensor can be irradiated on a top surface, and in a second embodiment, the substrate body is made sufficiently thin that the sensor can be irradiated on a bottom surface. There is a problem in using this image sensor in certain applications; for example, the sensor cannot be used in apparatus where it is desired to simultaneously image two opposing surfaces. In such an application, two separate image sensors must be used, and this adds to the expense and complexity of the apparatus.
U.S. Pat. No. 4,665,420, discloses a CCD image sensor which is adapted to receive illumination on a top surface. In order to prevent the injection of undesirable charge carriers into the CCD registers, the image sensor includes a means of passivating the edges of the sensor. The image sensor includes a plurality of space detectors arranged in columns extending along one of the major surfaces of the sensor substrate. Between the edge of the substrate and the adjacent column of detectors, an edge drain is provided for receiving any charge carriers generated at the edge in order to prevent the charge carriers from being injected into the adjacent detectors. Although charge carriers are being collected in this image sensor from an edge of the sensor, there is no provision for using the charge carriers to record information. Thus, the sensor can only be used to image in a single plane.